1. Field of the Invention
The present invention relates to an arc start method in a consumable electrode type arc welding method and, more particularly relates to an arc start method improving an arc start characteristic.
2. Description of the Related Art
In a related art, there is known a method of automatically carrying out arc welding by using a welding robot or the like. It is important in the automatic welding to firmly execute arc start. Therefore, there have been proposed various arc start methods in order to firmly carry out arc start (refer to, for example, Patent Reference 1, Patent Reference 2, Patent Reference 3, Patent Reference 4).
[Patent Reference 1] JP-A-7-9141
[Patent Reference 2] JP-A-9-253852
[Patent Reference 3] JP-A-7-185816
[Patent Reference 4] JP-A-11-347732
Meanwhile, according to the arc start methods disclosed in the respective patent references, when welding speed is slow, a serious problem is not posed. However, it becomes apparent that when the welding speed becomes a high speed of, for example, about 3 m per minute, a problem resulting from instability of arc start is posed. When investigated on the problem, the following knowledge is acquired. That is, according to an arc start instable state in arc welding, as shown by FIG. 1, there is brought about an arc interrupting phenomenon in which arc is temporarily extinguished within several 100 mS (milliseconds) after arc start and, thereafter, arc is reignited. In arc welding at normal speed, the welding speed is slow, a welding length per second is short (for example, when the welding speed is 60 cm/minute, a welding length per second is 1 cm) and therefore, even when the extinguishing and reigniting phenomenon is brought about, the phenomenon is difficult to emerge as a welding defect of an irregular bead, a deficiency in melting or the like. However, when the welding speed is fast, and the welding length per second is long (for example, when the welding speed is 3 in/minute, the welding length per second is 5 cm), the arc interrupting phenomenon of extinguishing and reigniting is easy to emerge as the welding defect of the irregular bead, the deficiency in melting or the like.
Hence, the inventors have investigated the reason of bringing about the arc interrupting phenomenon of extinguishing and reigniting. Generally, arc start in a consumable electrode type arc welding method is carried out by feeding a welding wire at a low speed and making the welding wire shortcircuit with a base metal. At this occasion, in the welding wire, an excessively large start current resulting from the shortcircuit flows instantaneously. Then, a portion of the welding wire projected from a welding tip is instantaneously and totally heated. When wire feeding thereafter is retarded, the arc length becomes excessively long, thereby extinguishing the arc, or the shortcircuit is not opened and a shortcircuit phenomenon continues over a long period of time and, by excessively heating the welding wire, the welding wire is melted to break at a middle thereof, or totally at the projected portion, a front end side thereof is blown by an arc force and arc is temporarily extinguished. Thereafter, arc is generated again at a new front end portion of the welding wire formed by shortcircuiting the wire again. Further, although in the case of the instantaneous arc interrupting phenomenon, it seems macroscopically as if the arc were continued as a result, depending on arc interrupting time, the phenomenon may emerge as a welding defect. Particularly, the arc interrupting phenomenon in high speed welding is liable to emerge as the welding defect of the irregular bead or the like.